Positive-electrode active material for lithium-ion secondary battery, positive electrode, manufacturing method thereof, and lithium-ion secondary battery

ABSTRACT

A positive-electrode active material for a lithium-ion secondary battery has an average composition expressed by the following formula (1): 
       Li x Co 1−y−z M y Ce z O b−a X a    (1) 
     wherein M represents at least one element selected from the group consisting of boron B, magnesium Mg, aluminum Al, silicon Si, phosphorous P, sulfur S, titanium Ti, chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu, zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag, tungsten W, indium In, tin Sn, lead Pb, and antimony Sb, X represents a halogen element, and x, y, z, a, and b satisfy 0.2&lt;x≦1.2, 0≦y≦0.1, 0.5&lt;z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0, respectively, and the concentration of cerium Ce is higher in the vicinity of the surface than in the inside.

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Japanese Patent ApplicationJP 2007-295177 filed in the Japanese Patent Office on Nov. 14, 2007, theentire contents of which is incorporated herein by reference.

BACKGROUND

The present application relates to a positive-electrode material for alithium-ion secondary battery, a positive electrode, a method ofmanufacturing the positive electrode, and a lithium-ion secondarybattery employing the positive electrode.

With the recent increase in performance and multi-functionality ofmobile instruments, an increase in capacity of a secondary battery as apower source thereof has been urgently desired. As the secondary batterythat can satisfy the desire, a nonaqueous-electrolyte secondary batteryusing lithium cobaltate for a positive electrode, using graphite for anegative electrode, and an organic mixture solvent containing a lithiumsalt supporting electrolyte for an electrolyte has attracted attention.

In the nonaqueous-electrolyte secondary battery working with 4.2 V inmaximum, a positive-electrode active material such as lithium cobaltateused for the positive electrode provides only about 60% capacity of itstheoretical capacity for actual use. Accordingly, by further raising acharging voltage, the remaining capacity can be utilized in principle.Actually, it is known that it is possible to enhance the energy densityby setting the charging upper limit voltage to 4.25 V or more (forexample, see PCT Publication No. 03/019713). In order to cope with thenew requirement for an increase in capacity, the high-capacity negativeelectrode employing silicon Si, germanium Ge, tin Sn, and the like hasbeen actively studied in recent years.

The above-mentioned nonaqueous-electrolyte secondary battery is mainlyused in mobile instruments such as notebook personal computers andmobile phones and is exposed to relatively high temperature due to heatemitted from the instruments or heat inside a moving vehicle for a longtime. When the charged nonaqueous-electrolyte secondary battery isexposed to such an environment, gas might be generated from a reactionof the positive electrode and the electrolyte.

When the gas is generated and for example, when thenonaqueous-electrolyte secondary battery is housed in a sheath memberformed of a laminate film, the sheath member is inflated to enhance thethickness thereof and thus is not fit to the specification of a batteryhousing of an electronic apparatus. The internal resistance of thebattery increases due to the reaction of the positive electrode and theelectrolyte, thereby not utilizing the sufficient capacity.

Such a phenomenon causes a problem in the batteries with the pastoperating voltage and remarkably occurs in batteries of which the upperlimit voltage is set to 4.25 V or more. It is considered that this isbecause the potential of the positive electrode increases in comparisonwith the past system and thus the reactivity to the electrolyte ispromoted. Similarly, such a phenomenon causes a problem in high-capacitybatteries using silicon Si, germanium Ge, or tin Sn for the negativeelectrode. This is because the potential of the negative electrode ishigher than the past graphite negative electrode. Accordingly, even whenit is used with the past operating voltage, the potential of thepositive electrode increases in comparison with the past system.

There is a problem with a cycle characteristic in the batteries usingthe negative electrodes. However, there has been suggested that anelectrolyte containing fluorine in molecules is used, thereby greatlyimproving the cycle characteristic.

However, the fluorine-based electrolyte is decomposed by the positiveelectrode at the time of conservation at a high temperature, therebypromoting the generation of gas.

On the other hand, in order to solve the above-mentioned problems,inventors of the present application has suggested a method of using apositive-electrode active material of which the surface is coated withanother compound or coating the surface with a compound at the time ofmanufacturing a positive-electrode coating slurry to form a stablesurface layer and thus to suppress the reactivity to the electrolyte(JP-A-2007-3 35331).

SUMMARY

As a result of the inventors' further study, improvements can be madebased on inventors' earlier findings as discussed above.

That is, by performing the processes as described above, it was foundthat the nature and status of the positive-electrode mixture slurry varyat the time of manufacturing the electrode, thereby making it difficultto coat a current collector with the slurry. Specifically, since carbonof a conductive material in the positive-electrode mixture slurrycoheres to deteriorate the fluidity of the slurry, the amount of addeddispersion medium should be enhanced. On the contrary, when the amountof added dispersion medium is enhanced, the separation easily occursduring the storage of the slurry.

When a large amount of dispersion medium is used, the cost for rawmaterials increases and a large amount of heat is needed for drying theslurry after applying the slurry, thereby deteriorating theproductivity. It was also proved that the distribution of the conductivematerial and the binder in the dried mixture layer is deviated and thusthe electrode mixture layer is easily separated from the currentcollector. Accordingly, there is a problem in that the mixture layer isdropped at the time of winding or the capacity is deteriorated due tothe long-term cycle of the battery.

There is a need for providing a positive-electrode active material for alithium-ion secondary battery that has an excellent high-temperaturestorage characteristic and that is excellent in uniformity, durability,and productivity of a positive-electrode mixture layer, a positiveelectrode employing the positive-electrode active material, a method ofmanufacturing the positive electrode, and a lithium-ion secondarybattery employing the positive electrode.

Upon further evaluation, the inventors discovered that theabove-mentioned need can be accomplished by combining predeterminedlithium composite oxide, in which a lot of cerium Ce exists in thevicinity of the surface, with a compound containing sulfur S orphosphorous P as needed.

That is, according to an embodiment, there is provided apositive-electrode active material for a lithium-ion secondary batteryhaving an average composition expressed by the following formula (1):

Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1)

wherein M represents at least one element selected from the groupconsisting of boron B, magnesium Mg, aluminum Al, silicon Si,phosphorous P, sulfur S, titanium Ti, chromium Cr, manganese Mn, ironFe, cobalt Co, nickel Ni, copper Cu, zinc Zn, gallium Ga, yttrium Y,zirconium Zr, molybdenum Mo, silver Ag, tungsten W, indium In, tin Sn,lead Pb, and antimony Sb, X represents a halogen element, and x, y, z,a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1, 0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0,respectively. Here, the concentration of cerium Ce is higher in thevicinity of the surface than in the inside.

According to another embodiment, there is provided a positive electrodefor a lithium-ion secondary battery including a positive-electrodemixture layer and a current collector. The positive-electrode mixturelayer includes a positive-electrode active material, a conductivematerial, and a binder. Here, the positive-electrode active material hasan average composition expressed by the following formula (1):

Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1)

wherein M represents at least one element selected from the groupconsisting of boron B, magnesium Mg, aluminum Al, silicon Si,phosphorous P, sulfur S, titanium Ti, chromium Cr, manganese Mn, ironFe, cobalt Co, nickel Ni, copper Cu, zinc Zn, gallium Ga, yttrium Y,zirconium Zr, molybdenum Mo, silver Ag, tungsten W, indium In, tin Sn,lead Pb, and antimony Sb, X represents a halogen element, and x, y, z,a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1, 0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0,respectively. The concentration of cerium Ce is higher in the vicinityof the surface than in the inside. A compound including sulfur S and/orphosphorous P is included in the positive-electrode mixture layer.

According to another embodiment, there is provided a method ofmanufacturing a positive electrode for a lithium-ion secondary batteryincluding a positive-electrode mixture layer, which includes apositive-electrode active material, a conductive material, and a binder,and a current collector. Here, the positive-electrode active materialhas an average composition expressed by the following formula (1):

Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1)

wherein M represents at least one element selected from the groupconsisting of boron B, magnesium Mg, aluminum Al, silicon Si,phosphorous P, sulfur S, titanium Ti, chromium Cr, manganese Mn, ironFe, cobalt Co, nickel Ni, copper Cu, zinc Zn, gallium Ga, yttrium Y,zirconium Zr, molybdenum Mo, silver Ag, tungsten W, indium hi, tin Sn,lead Pb, and antimony Sb, X represents a halogen element, and x, y, z,a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1, 0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0,respectively. The concentration of cerium Ce is higher in the vicinityof the surface than in the inside. A compound containing sulfur S and/orphosphorous P is included in the positive-electrode mixture layer. Themethod includes the step of adding the compound containing sulfur Sand/or phosphorous P to a positive-electrode mixture slurry used to formthe positive-electrode mixture layer.

According to still another embodiment, there is provided a lithium-ionsecondary battery including a positive electrode, which includes apositive-electrode mixture layer having a positive-electrode activematerial, a conductive material, and a binder, and a current collector,a negative electrode, and an electrolyte. Here, the positive-electrodeactive material has an average composition expressed by the followingformula (1):

Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1)

wherein M represents at least one element selected from the groupconsisting of boron B, magnesium Mg, aluminum Al, silicon Si,phosphorous P, sulfur S, titanium Ti, chromium Cr, manganese Mn, ironFe, cobalt Co, nickel Ni, copper Cu, zinc Zn, gallium Ga, yttrium Y,zirconium Zr, molybdenum Mo, silver Ag, tungsten W, indium In, tin Sn,lead Pb, and antimony Sb, X represents a halogen element, and x, y, z,a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1, 0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0,respectively. The concentration of cerium Ce is higher in the vicinityof the surface than in the inside. A compound including sulfur S and/orphosphorous P is included in the positive-electrode mixture layer.

According to the above-mentioned embodiments, since the predeterminedlithium composite oxide in which a lot of cerium Ce exists in thevicinity of the surface is combined with the compound including sulfur Sand/or phosphorous P as needed, it is possible to provide apositive-electrode active material for a lithium-ion secondary batterythat is excellent in uniformity, durability, and productivity of thepositive-electrode mixture layer, a positive electrode employing thepositive-electrode active material, a method of manufacturing thepositive electrode, and a lithium-ion secondary battery employing thepositive electrode.

Additional features and advantages are described herein, and will beapparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a perspective view illustrating a configuration of a secondarybattery according to a first embodiment.

FIG. 2 is a sectional view of a battery element 10 taken along lineII-II of FIG. 1.

FIG. 3 is a sectional view illustrating a configuration of a secondarybattery according to a third embodiment.

FIG. 4 is a partially-enlarged sectional view illustrating a woundelectrode member 30 shown in FIG. 3.

DETAILED DESCRIPTION

Hereinafter, a positive-electrode active material for a lithium-ionsecondary battery, a positive electrode employing the positive-electrodeactive material, a method of manufacturing the positive electrode, and alithium-ion secondary battery employing the positive electrode accordingto exemplary embodiments will be described in detail with reference tothe figures. In the following description, “%” associated withconcentration, content, and mixed amount represents a mass percentage aslong as it is not particularly described differently.

1 First Embodiment 1-1 Positive-Electrode Active Material

A positive-electrode active material according to a first embodimentincludes a lot of cerium Ce in the vicinity of the surface and theconcentration of cerium is higher in the vicinity of the surface than inthe inside thereof. Typically, the positive-electrode active material isparticle shaped and a surface layer having a high concentration ofcerium is formed in at least a part of the surface of a predeterminedlithium composite oxide particle as a center thereof

An average composition of the lithium composite oxide is expressed bythe following formula (1):

Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1)

wherein M represents at least one element selected from the groupconsisting of boron B, magnesium Mg, aluminum Al, silicon Si,phosphorous P, sulfur S, titanium Ti, chromium Cr, manganese Mn, ironFe, cobalt Co, nickel Ni, copper Cu, zinc Zn, gallium Ga, yttrium Y,zirconium Zr, molybdenum Mo, silver Ag, tungsten W, indium In, tin Sn,lead Pb, and antimony Sb, X represents a halogen element, and x, y, z,a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1, 0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0,respectively.

The surface layer serves as a reaction suppressing layer and containscerium Ce. The concentration of cerium therein is higher than thatinside the lithium composite oxide particle.

The positive-electrode active material can be obtained by mixing andsintering a lithium-source compound, a cobalt-source compound, acompound containing other additive elements, and a cerium-sourcecompound at the time of synthesizing a lithium composite compound.

An example of the lithium source includes lithium carbonate, lithiumhydroxide, and lithium nitrate. An example of the cobalt source includescobalt oxide, cobalt carbonate, and cobalt oxyhydroxide. An example ofthe cerium source includes cerium oxide and cerium nitrate.

At the time of performing the above-mentioned sintering, cerium havingan ion radius greatly different from that of cobalt ions is not solvedand is educed on the surface, thereby forming the surface layer.

In another method, the surface layer can be obtained by attaching acerium compound to the surface of a lithium composite compound notcontaining cerium. An example of the attachment method includes amechanofusion method, a mechanohybridization method, and a spray drymethod. By heating the attached particles, the adhesion between thesurface layer and the composite oxide particles can be made to bestrong.

A method of confirming whether cerium exists in the particle surface ofthe lithium composite oxide is the same as a method for the compoundcontaining sulfur S and/or phosphorous P to be described later.

The content of cerium in the vicinity of the surface, particularly, inthe surface layer, is preferably in the range of 5 to 50 at. % withrespect to cobalt Co in the lithium composite oxide.

When the content is less than 5 at. %, the reaction of the surface ofthe active material and the electrolyte solution at the time ofhigh-temperature storage cannot be sufficiently suppressed. When thecontent is greater than 50 at. %, the insertion of lithium ions into theactive material particles is hindered, thereby deteriorating the loadcharacteristic.

1-2 Positive-Electrode Mixture (Layer)

The positive-electrode mixture includes the positive-electrode activematerial, a conductive material, and a binder and contains compoundsincluding sulfur S and/or phosphorous P in an embodiment. Preferably,the compounds much exist in the vicinity of the particle surface of thelithium composite oxide, and typically cover at least a part of theparticle surfaces to form a surface layer.

The surface layer serves as a reaction suppressing layer and the contentof at least one of sulfur S and phosphorous P in the vicinity of theparticle surface of the lithium composite oxide, particularly, in thesurface layer, is higher than that inside the lithium composite oxideparticles.

The materials are contained, for example, as a compound in the surfacelayer. More specifically, sulfur S is contained, for example, as Li₂SO₄in the surface layer and phosphorous P is contained, for example, asLi₃PO₄ or LiCoPO₄ in the surface layer.

The content of at least one of sulfur S and phosphorous P is preferablyequal to or more than 0.02 at. % and less than 10 at. % with respect tocobalt Co in the positive-electrode active material. When the content isequal to or more than 0.02 at. %, it is possible to obtain an excellentstorage characteristic. When the content is less than 10 at. %, it ispossible to suppress an increase in internal resistance or a decrease incapacity.

The positive-electrode active material can be obtained by adding lithiumcomposite oxide particles to an aqueous solution containing at least oneof a sulfur-containing compound and a phosphorous-containing compoundand kneading and then drying the resultant. For example, a hot-windfixed shelf drier or a spray drier may be used as the dry method. Theobtained dried mixture may be heated to stabilize the surface products.

One or two or more sulfur-containing compounds can be used as thesulfur-containing compound. An example of the sulfur-containing compoundincludes sulfate, sulfite, ammonium sulfate, ammonium hydrogen sulfate,and organic sulfate. One or two or more phosphorous-containing compoundscan be used as the phosphorous-containing compound. An example of thephosphorous-containing compound includes phosphate, phosphite,hypophosphite, ammonium phosphate, ammonium hydrogen phosphate, andorganic phosphate. In this embodiment, phosphonic acid, methane sulfonicacid, sulfobenzoic acid, sulfobenzoic anhydride, and mixtures thereofcan be suitably used.

As a method of confirming that cerium Ce or at least one of sulfur S andphosphorous P exists in the surface, a method of comparing the atomicratio and the feed composition of at least one of sulfur S andphosphorous P existing in the surface with respect to cobalt Co in thesurface analysis of the scanning electron microscopy-energy dispersionX-ray spectrometry (SEM-EDS) and the X-ray photoelectron spectroscopy(XPS) can be used.

The positive-electrode active material is embedded in resin, thesectional intrusion is performed, and then the distribution in thesection can be confirmed by the use of the time-of-flight secondary ionmass spectroscopy (TOF-SIMS). The surface compounds can be identified bythe use of the X-ray diffraction (XRD) measurement or the TOF-SIMSmeasurement.

By the use of the above-mentioned analysis methods, particularly, theSEM-EDS, it can be confirmed whether cerium or sulfur S and/orphosphorous P forms the surface layer in all or a part of the particlesurfaces of lithium composite oxide.

The mean particle size of the lithium composite oxide is preferablyequal to or more than 1 μm and less than 30 μm. When the mean particlesize is equal to or more than 1 μm it is possible to suppress thereaction of the positive electrode and the electrolyte, therebysuppressing the increase in generated gas. When the mean particle sizeis less than 30 μm, it is possible to obtain the sufficient capacity orthe excellent load characteristic.

As described above, in this embodiment, the surface layer of compoundsincluding cerium Ce or sulfur S and/or phosphorous P is preferablyformed in predetermined lithium composite oxide in thepositive-electrode active material or the positive-electrode mixturelayer.

As described above, it is considered that cerium Ce forms a stablecoating on the surface of the active material to suppress the reactionto the electrolyte solution. By adding compounds including at least oneof phosphorous P and sulfur S to the positive-electrode mixture slurry,a thin coating is formed on the particle surface of lithium compositeoxide. Since the coating has low activity about the electrolyte, it isconsidered that it is possible to suppress the reaction of the positiveelectrode and the electrolyte solution in the high-temperature storagestate and to suppress the generation of gas or the increase in internalresistance.

In reforming the surface of the active material using cerium, when theamount of cerium increases, the high-temperature storage characteristicis improved depending on the added amount, but the battery capacity perweight is deteriorated. Accordingly, the maximum amount that can beactually added thereto is limited.

On the other hand, in reforming the surface using the compoundsincluding at least one of phosphorous P and sulfur S, when the amount ofthe compounds increases, the nature and state of the mixture slurry aremarkedly deteriorated. Accordingly, by adding the compounds including atleast one of phosphorous P and sulfur S to the positive-electrodemixture slurry in minimum using the positive-electrode active materialof which the surface is reformed by cerium, it is possible to compensatefor both defects and to improve the high-temperature storagecharacteristic.

1-3 Configuration of Secondary Battery

A configuration of a secondary battery according to the first embodimentwill be described now with reference to FIGS. 1 and 2.

FIG. 1 is a perspective view illustrating a configuration of thesecondary battery according to the first embodiment.

The secondary battery has a configuration in which a battery element 10to which a positive electrode lead 11 and a negative electrode lead 12are attached is housed in a film-like sheath member 1.

The positive electrode lead 11 and the negative electrode lead 12 have alongitudinal shape and are drawn out in the same direction toward theoutside from the inside of the sheath member 1. The positive electrodelead 11 is formed of a metal material such as aluminum and the negativeelectrode lead 12 is formed of a metal material such as nickel Ni.

The sheath member 1 has a structure in which an insulating layer, ametal layer, and an outermost layer are laminated in this order and arebonded by a lamination process or the like. In the sheath member 1, theinsulating layer is located on the inside and the outer edges areclosely adhered to each other by fusion or adhesive.

The insulating layer is formed of polyolefin resin such as polyethylene,polypropylene, denatured polyethylene, denatured polypropylene, andcopolymers thereof. These materials can lower the moisture transmissionproperty and are excellent air-tightness.

The metal layer is formed of foil-like or plate-like aluminum,stainless, nickel, or iron. The outermost layer may be formed of thesame resin as the insulating layer or nylon. These materials can enhancethe resistance to the teardown or lunge. The sheath member 1 may havelayers other than the insulating layer, the metal layer, and theoutermost layer.

An adhesion film 2 for improving the close adhesion between the positiveelectrode lead 11 and the negative electrode lead 12 and the inside ofthe sheath member 1 to prevent the invasion of ambient air is insertedbetween the sheath member 1 and the positive electrode lead 11 and thenegative electrode lead 12. The adhesion film 2 is formed of a materialhaving a close adhesion property to the positive electrode lead 11 andthe negative electrode lead 12, and is preferably formed of polyolefinresin such as polyethylene, polypropylene, denatured polyethylene, anddenatured polypropylene, when the positive electrode lead 11 and thenegative electrode lead 12 are formed of the above-mentioned metalmaterial.

FIG. 2 is a sectional view of the battery element 10 taken along lineII-II of FIG. 1.

The battery element 10 is obtained by stacking and winding the positiveelectrode 13 and the negative electrode 14 with the separator 15 and theelectrolyte 16 interposed therebetween, and the outermost portion isprotected by a protective tape 17.

The positive electrode 13 includes a positive-electrode currentcollector 13A and positive-electrode mixture layers 13B disposed on bothsides of the positive-electrode current collector 13A. Thepositive-electrode current collector 13A is formed of a metal foil suchas an aluminum foil.

The positive-electrode mixture layer 13B includes the positive-electrodeactive material, a conductive material such as carbon if necessary, abinder such as polyvinylidene fluoride or polytetrafluoroethylene.

The positive-electrode mixture layer 13B may further include anotherpositive-electrode active material in addition to the above-mentionedpositive-electrode active material. An example of the anotherpositive-electrode active material includes lithium-nickel compositeoxide including lithium and nickel, lithium-manganese composite oxidehaving a spinel structure including lithium and manganese, and phosphatecompound including lithium and iron.

The negative electrode 14 includes a negative-electrode currentcollector 14A and a negative-electrode mixture layer 14B disposed onboth sides of the negative-electrode current collector 14A, similarly tothe positive electrode 13. The negative-electrode current collector 14Ais formed of a metal foil such as a copper foil.

The negative-electrode mixture layer 14B includes one or two or morenegative electrode materials, which can occlude and desorb lithium, asthe negative-electrode active material, and may further include aconductive material and a binder as needed.

An example of the negative electrode material that can occlude anddesorb lithium includes carbon materials such as graphite,non-graphitizable carbon, and graphitized carbon. One of the carbonmaterials may be used singly or two or more may be mixed. Alternatively,two or more having different average particle diameters may be mixed foruse.

An example of the negative electrode material that can occlude anddesorb lithium includes a material including metal element or semimetalelement forming an alloy along with lithium. Specifically, examples ofthe negative electrode material include a single body, an alloy, and acompound of the metal element forming an alloy along with lithium, asingle body, an alloy, and a compound of the semimetal element formingan alloy along with lithium, and materials having phases of one or twoor more thereof in at least a part.

An example of the metal element or semimetal element includes tin Sn,lead Pb, aluminum Al, indium In, silicon Si, zinc Zn, antimony Sb,bismuth Bi, cadmium Cd, magnesium Mg, boron B, gallium Ga, germanium Ge,arsenic As, silver Ag, zirconium Zr, yttrium Y, and hafnium Hf. Metalelements or semimetal elements of Group 14 in the long-period periodictable are preferable and silicon Si or tin Sn is particularlypreferable. Silicon Si and tin Sn have great ability of occluding anddesorbing lithium and provide high energy density.

The silicon alloy includes at least one selected from the groupconsisting of tin Sn, nickel Ni, copper Cu, iron Fe, cobalt Co,manganese Mn, zinc Zn, indium In, silver Ag, titanium Ti, germanium Ge,bismuth Bi, antimony Sb, and chromium Cr as a second element other thansilicon. The tin Sn alloy includes at least one selected from the groupconsisting of silicon Si, nickel Ni, copper Cu, iron Fe, cobalt Co,manganese Mn, zinc Zn, indium In, silver Ag, titanium Ti, germanium Ge,bismuth Bi, antimony Sb, and chromium Cr as a second element other thantin Sn.

The compound of silicon Si or the compound of tin Sn includes, forexample, oxygen O or carbon C, and may further include theabove-mentioned second elements in addition to silicon Si or tin Sn.

The separator 15 is formed of any material, as long as it iselectrically stable and chemically stable about the positive-electrodeactive material, the negative-electrode active material, and thesolvent, and does not have the electrical conductivity. For example, anunwoven fabric of polymer, a porous film, a paper-like fiber of glass orceramics can be used and plural ones thereof may be stacked.Particularly, a porous polyolefin film is preferably used or a compositethereof with heat-resistance materials such as fibers of polyimide,glass, and ceramics may be used.

The electrolyte 16 includes an electrolyte solution and a holding membercontaining polymer compounds for holding the electrolyte solution and isof a gel type. The electrolyte includes an electrolyte salt and asolvent dissolving the electrolyte salt. An example of the electrolytesalt includes lithium salts such as LiClO₄, LiPF₆, LiBF₄, LiN(SO₂CF₃)₂,LiN(SO₂C₂F₅)₂, and LiAsF₆. One thereof may be used as the electrolytesalt or two or more thereof may be mixed for use.

An example of the solvent includes lactone-based solvent such asγ-butyrolactone, γ-valerolactone, δ-valerolactone, and ε-caprolactone,ester carbonate-based solvent such as ethylene carbonate, propylenecarbonate, butylenes carbonate, vinylene carbonate, dimethyl carbonate,ethylmethyl carbonate, and diethyl carbonate, ether-based solvent suchas 1,2-dimethoxy ethane, 1-ethoxy-2-methoxy ethane, 1,2-diethoxy ethane,tetrahydrofuran, and 2-methyltetrahydrofuran, nitrile-based solvent suchas acetnitrile, sulforan-based solvent, phosphates, ether phosphatesolvent, and nonaqueous solvent such as pyrrolidones. One thereof may beused singly as the solvent or two or more may be mixed for use.

The solvent preferably includes a cyclic ester or a chained ester inwhich all or a part of hydrogen is fluorinated. The fluorinated compoundpreferably includes fluoroethylene carbonate or difluoroethylenecarbonate.

When the negative electrode 14 including compounds of silicon Si, tinSn, and germanium Ge as the negative-electrode active material is used,it is possible to improve the charging and discharging cyclecharacteristic.

The polymer compound serves to absorb and gelate the solvent and anexample thereof includes fluorine-based polymer compounds such aspolyvinylidene fluoride or vinylidene fluoride and copolymer ofhexafluoropropylene, ether-based polymer compounds such as crosslinkerincluding polyethylene oxide or polyethylene oxide, and polymercompounds including polyacrylonitrile, polypropylene oxide, orpolymethyl methacrylate as a repetition unit. One of the first polymercompounds may be used singly or two or more may be mixed for use.

Particularly, the fluorine-based polymer compounds are preferable inview of stability of oxidation and reduction and copolymers containingvinylidene fluoride and hexafluoropropylene are more preferable. Thecopolymers may contain monoester of unsaturated dibasic acid such asmonomethyl ester maleate, ethylene halide such as triethylenefluorochloride, a cyclic ester carbonate of an unsaturated compound suchas vinylene carbonate, or acrylvinyl monomer containing epoxy group.This is because the polymers provide higher characteristic.

In the secondary battery having the above-mentioned configuration, anopen-circuit voltage in a fully-charged state of a pair of positiveelectrode and negative electrode is set to, for example, 4.2 V orhigher. The open-circuit voltage set to be higher than 4.2 V ispreferably in the range of 4.25 V to 4.6 V and more preferably in therange of 4.35 V to 4.6 V. When the upper-limit charging voltage of thesecondary battery is raised, the utilization rate of thepositive-electrode active material can be enhanced, thereby extractingmore energy. When the open-circuit voltage is 4.6 V or less, theoxidation of the separator 15 can be suppressed.

1-4 Method of Manufacturing Secondary Battery

A method of manufacturing the secondary battery according to the firstembodiment will be described now.

First, the positive-electrode mixture layer 13B is formed on thepositive-electrode current collector 13A to produce the positiveelectrode 13. The positive-electrode mixture layer 13B is formed bymixing powder of the positive-electrode active material, the conductivematerial and the binder to prepare a positive-electrode mixture,dispersing the positive-electrode mixture in a solvent such asN-methyl-2-pyrrolidone to form a paste-like positive-electrode mixtureslurry, applying the positive-electrode mixture slurry to thepositive-electrode current collector 13A, and drying and compacting theresultant.

The resultant obtained by adding phosphonic acid or methane sulfonicacid to a slurry including lithium composite oxide instead of thepositive-electrode active material coated with sulfur and/or phosphorousmay be used as the positive-electrode mixture slurry.

On the other hand, similarly to the positive electrode 13, thenegative-electrode mixture layer 14B is formed on the negative-electrodecurrent collector 14A to form the negative electrode 14. Then, thepositive electrode lead 11 is attached to the positive-electrode currentcollector 13A and the negative electrode lead 12 is attached to thenegative-electrode current collector 14A.

Then, the electrolyte and the polymer compound are mixed with a mixturesolvent, the mixture solution is applied to the positive-electrodemixture layer 13B and the negative-electrode mixture layer 14B, and themixture solvent is volatilized to form the electrolyte 16. The positiveelectrode 13, the separator 15, the negative electrode 14, and theseparator 15 are sequentially stacked and wound, a protective tape 17 isattached to the outermost circumferential portion to form the batteryelement 10, the battery element is inserted into the sheath member 1,and then the outer circumferential edge of the sheath member 1 isthermally fused and bonded. At this time, the adhesion film 2 isinserted between the positive electrode lead 11 and the negativeelectrode lead 12 and the sheath member 1. As a result, the secondarybattery shown in FIG. 1 is obtained.

The positive electrode 13 and the negative electrode 14 are wound withthe separator 15 interposed therebetween instead of winding the positiveelectrode 13 and the negative electrode 14 after the electrolyte 16 isformed thereon, the resultant structure is inserted into the sheathmember 1, and an electrolyte composition including an electrolytesolution and a monomer of the polymer compounds is injected therein,thereby polymerizing the monomer in the sheath member 1.

In the secondary battery, when it is charged, lithium ions are emittedfrom the positive electrode 13 and are occluded in the negativeelectrode 14 through the electrolyte 16. On the other hand, when it isdischarged, lithium ions are emitted from the negative electrode 14 andoccluded in the positive electrode 13 through the electrolyte 16.

As described above, according to the first embodiment, the surface layercontaining cerium Ce is formed in at least a part of the respectivelithium composite oxide particles as a center. Accordingly, when thecharged secondary battery is stored under a high-temperature condition,it is possible to prevent gas from being generated due to the reactionof the positive electrode 31 and the electrolyte. It is also possible toprevent the internal resistance from increasing due to the reaction ofthe positive electrode 31 and the electrolyte.

Since the surface layer also includes at least one of phosphorous P andsulfur S, the reaction to the electrolyte solution is furthersuppressed.

Even when the open-circuit voltage in the fully-charged state of a pairof positive electrode and negative electrode is set to the range of 4.25V, which is greater than 4.2 V, to 4.6 V or the range of 4.35 V to 4.6 Vand the utilization rate of the positive-electrode active material isenhanced to raise the potential of the positive electrode 13, it ispossible to suppress gas from being generated due to the reaction of thepositive electrode 13 and the electrolyte. That is, it is possible toextract more energy and to remarkably improve the high-temperaturestorage characteristic.

Even in a secondary battery employing graphite widely used in the pastfor the negative-electrode active material, it is possible to suppressthe increase in thickness of the battery at the time of high-temperaturestorage. This suppression is further remarkable in the secondary batteryemploying the negative electrode 14 using compounds such as silicon Si,tin Sn, and germanium Ge for the negative-electrode active material andemploying cyclic or chained ester fluoride for the electrolyte 16.

2 Second Embodiment

A second embodiment will be described now. A secondary battery accordingto the second embodiment employs an electrolyte solution instead of thegel electrolyte 16 in the secondary battery according to the firstembodiment. In this case, the electrolyte solution is impregnated in theseparator 15. The same as the first embodiment can be used as theelectrolyte solution.

The secondary battery having the above-mentioned configuration can bemanufactured as follows. Similarly to the first embodiment except thatthe formation of the gel electrolyte 16 is omitted, the positiveelectrode 13 and the negative electrode 14 are wound to form a batteryelement 10, the battery element 10 is inserted into the sheath member 1,the electrolyte solution is injected therein, and then the sheath member1 is sealed.

According to the second embodiment, it is possible to obtain the sameadvantages as the first embodiment.

3 Third Embodiment 3-1 Positive-Electrode Active Material

The positive-electrode active material in a third embodiment is similarto that of the first embodiment and thus description thereof is omitted.

3-2 Configuration of Secondary Battery

A configuration of a secondary battery according to the third embodimentwill be described now with reference to FIGS. 3 and 4.

FIG. 3 is a sectional view illustrating a configuration of the secondarybattery according to the third embodiment.

The secondary battery is called a cylindrical type and has a woundelectrode member 30, in which a band-like positive electrode 31 and aband-like negative electrode 32 are wound with a separator 33 interposedtherebetween, in a substantially hollow column-like battery can 21. Theelectrolyte solution as a liquid electrolyte is impregnated in theseparator 33. The battery can 21 is formed of iron Fe coated with nickelNi, where one end is closed and the other end is opened. In the batterycan 21, a pair of insulating plates 22 and 23 is disposed perpendicularto the winding circumferential surfaces with the wound electrode member30 interposed therebetween.

A battery lid 24 and a safety valve mechanism 25 and a heat-sensitiveresistor (PTC: Positive Temperature Coefficient) element 26 disposedinside the battery lid 24 are attached to the open end of the batterycan 21 by caulking with a gasket 27 and the inside of the battery can 21is sealed air-tightly. The battery lid 24 is formed of the same materialas the battery can 21. The safety valve mechanism 25 is electricallyconnected to the battery lid 24 through the heat-sensitive resistorelement 26. When the internal voltage of the battery is higher than apredetermined value due to internal short-circuit or external heat, adisc plate 25A is inverted to tear down the electrical connectionbetween the battery lid 24 and the wound electrode member 30.

The heat-sensitive resistor element 26 restricts the current due to theincrease in resistance with the increase in temperature, therebypreventing the abnormal heating due to large current. The gasket 27 isformed of an insulating material and asphalt is applied to the surfacethereof.

The wound electrode member 30 is wound on a center pin 34. A positiveelectrode lead 35 formed of aluminum Al is connected to the positiveelectrode 31 of the wound electrode member 30 and a negative electrodelead 36 formed of nickel Ni is connected to the negative electrode 32.The positive electrode lead 35 is electrically connected to the batterylid 24 by the welding to the safety valve mechanism 25 and the negativeelectrode lead 36 is electrically connected to the battery can 21 by thewelding.

FIG. 4 is a partially-enlarged sectional view illustrating the woundelectrode member 30 shown in FIG. 3. The wound electrode member 30 isobtained by stacking and winding the positive electrode 31 and thenegative electrode 32 with the separator 33 interposed therebetween.

The positive electrode 31 includes a positive-electrode currentcollector 31A and a positive-electrode mixture layer 31B disposed onboth sides of the positive-electrode current collector 31A. The negativeelectrode 32 includes a negative-electrode current collector 32A and anegative-electrode mixture layer 32B disposed on both sides of thenegative-electrode current collector 32A. The configurations of thepositive-electrode current collector 31A, the positive-electrode mixturelayer 31B, the negative-electrode current collector 32A, thenegative-electrode mixture layer 32B, the separator 33, and theelectrolyte are similar to those of the positive-electrode currentcollector 13A, the positive-electrode mixture layer 13B, thenegative-electrode current collector 14A, the negative-electrode mixturelayer 14B, the separator 15, and the electrolyte in the firstembodiment.

3-3 Method of Manufacturing Secondary Battery

A method of manufacturing the secondary battery according to the thirdembodiment will be described now.

The positive electrode 31 is formed as follows. First, thepositive-electrode active material, the conductive material, and thebinder are mixed to prepare a positive-electrode mixture, thepositive-electrode mixture is dispersed in a solvent such as1-methyl-2-pyrrolidone to form a positive-electrode mixture slurry.Then, the positive-electrode mixture slurry is applied to thepositive-electrode current collector 31A, the solvent is dried, and theresultant is compacted by a roll press machine to form thepositive-electrode mixture layer 31B, thereby obtaining the positiveelectrode 31. The resultant obtained by adding phosphonic acid ormethane sulfonic acid to a slurry including lithium composite oxideinstead of the positive-electrode active material coated with sulfurand/or phosphorous may be used as the positive-electrode mixture slurry.

The negative electrode 32 is formed as follows. First, thenegative-electrode active material and the binder are mixed to prepare anegative-electrode mixture and the negative-electrode mixture isdispersed in the solvent such as 1-methyl-2-pyrrolidone to form anegative-electrode mixture slurry. Then, the negative-electrode mixtureslurry is applied to the negative-electrode current collector 32A, thesolvent is dried, and the resultant is compacted by the roll pressmachine to form the negative-electrode mixture layer 32B, therebyobtaining the negative electrode 32.

The positive electrode lead 35 is attached to the positive-electrodecurrent collector 31A by welding and the negative electrode lead 36 isattached to the negative-electrode current collector 32A by welding.Thereafter, the positive electrode 31 and the negative electrode 32 arewound with the separator 33 interposed between, the end of the positiveelectrode lead 35 is welded to the safety valve mechanism 25, the end ofthe negative electrode lead 36 are welded to the battery can 21, thewound positive electrode 31 and negative electrode 32 are housed in thebattery can 21 with a pair of insulating plates 22 and 23 interposedbetween.

After the positive electrode 31 and the negative electrode 32 are housedin the battery can 21, the electrolyte is injected into the battery can21 and is impregnated in the separator 33. Thereafter, the batter lid24, the safety valve mechanism 25, and the heat-sensitive resistorelement 26 are fixed to the open end of the battery can 21 by caulkingwith the gasket 27. In this way, the secondary battery shown in FIG. 3is manufactured.

According to the third embodiment, it is possible to obtain the sameadvantages as the first embodiment.

4 Fourth Embodiment

A secondary battery according to a fourth embodiment has the sameconfiguration as the first embodiment, except for the positive-electrodemixture layer. Elements equal to or corresponding to the those of thefirst embodiment are denoted by like reference numerals and signs.

The positive-electrode mixture layer 13B includes the positive-electrodeactive material containing at least one of sulfur S and phosphorous P inthe vicinity of the particle surface of lithium composite oxide, and thecontent of at least one of sulfur S and phosphorous P in the vicinity ofthe particle surface of lithium composite oxide is the highest in thepositive-electrode mixture layer 13B. At least one of sulfur S andphosphorous P is included, for example, as a compound in thepositive-electrode mixture layer 13B. More specifically, sulfur S iscontained, for example, as Li₂SO₄ in the positive-electrode mixturelayer 13B and phosphorous P is contained, for example, as Li₃PO₄ orLiCoPO₄ in the positive-electrode mixture layer 13B. The lithiumcomposite oxide is similar to that of the first embodiment and cerium Ceis included in the surface layer.

The content of at least one of sulfur S and phosphorous P is preferablyequal to or more than 0.1 at. % and less than 10 at. % with respect tocobalt Co in the positive-electrode active material. When the content isequal to or more than 0.1 at. %, it is possible to obtain an excellentstorage characteristic. When the content is less than 10 at. %, it ispossible to suppress the increase in internal resistance or the decreasein capacity.

The mean particle size of the positive-electrode active material ispreferably equal to or more than 1 μm and less than 30 μm. When the meanparticle size is equal to or more than 1 μm, it is possible to suppressthe reaction of the positive electrode and the electrolyte solution andthus to suppress the increase in gas generation. When the mean particlesize is less than 30 μm, it is possible to obtain the sufficientcapacity or the excellent load characteristic.

It is preferable that the specific surface area is equal to or more than0.1 m²/g and less than 1 m²/g. When the specific surface area is equalto or more than 0.1 m²/g, it is possible to obtain the sufficientcapacity and the excellent load characteristic. When the specificsurface area is less than 1 m²/g, it is possible to suppress thereaction of the positive electrode and the electrolyte solution and thusto suppress the increase in gas generation.

As a method of confirming that at least one of sulfur S and phosphorousP in the positive-electrode mixture layer 13B exists in the vicinity ofthe particle surface of lithium composite oxide, a method of embeddingthe positive electrode 13 in resin, performing the sectional intrusion,and then confirming the distribution in the section by the use of theTOF-SIMS can be used. By analyzing elements by the use of the XPS whilesputtering the surface of the positive electrode with argon, theconfirmation can be made.

The positive-electrode mixture layer 13B has the peaks of one or moresecondary ion fragments selected from positive secondary ions of Li₄PO₄,Li₂CoPO₄, Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and negative secondaryions of PO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, and CoP₂O₆H in thesurface analysis using the TOF-SIMS.

The positive-electrode mixture layer 13B can be formed as follows.

The positive-electrode active material, the conductive material, thebinder, and at least one of a sulfur-containing compound and aphosphorous-containing compound are mixed to prepare apositive-electrode mixture, the positive-electrode mixture is kneadingin N-methyl pyrrolidone as the dispersion medium to form paste-likepositive-electrode mixture slurry, the positive-electrode mixture slurryis applied to the positive-electrode current collector 13A, dried, andcompacted. In this way, the positive-electrode mixture layer 13B isformed. The sulfur-containing compound and the phosphorous-containingcompound can be the same as described in the first embodiment.

According to the fourth embodiment, it is possible to obtain the sameadvantages as the first embodiment.

EXAMPLES

The present application will be described now in more detail withreference to examples and comparative examples, but the presentapplication is not limited to the examples.

Example A

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=95:5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

Then, as the result of surface analysis of an element distribution ofthe manufactured positive-electrode active material with the SEM-EDS, itwas seen that the element ratio of cerium Ce is 31 at. %, the cerium Ceelement is detected at the higher mol ratio relative to the feed amount,and the cerium Ce element exists much in the surface of thepositive-electrode active material, that is, the concentration of Ce ishigher in the surface than in the inside.

Then, 96 wt % of the manufactured positive-electrode active material, 3wt % of polyvinylidene fluoride as the first polymer of the binder, and1 wt % of Ketj en black as the conductive material were kneaded inN-methyl pyrrolidone as the dispersion medium to form apositive-electrode mixture slurry. The slurry was applied to apositive-electrode current collector formed of aluminum with a thicknessof 30 μm, dried, and compacted by the use of the roll press machine toform a positive-electrode mixture layer, thereby manufacturing thepositive electrode.

Graphite as the negative-electrode active material and polyvinylidenefluoride (PVDF) as the binder were mixed at the mass ratio of 90:10 andthe resultant negative-electrode mixture was dispersed in1-methyl-2-pyrrolidone, thereby forming a negative-electrode mixtureslurry. Then, the negative-electrode mixture slurry was applied to thenegative-electrode current collector, dried to remove the solvent, andcompacted by the use of the roll press machine to form thenegative-electrode mixture layer, thereby manufacturing the negativeelectrode.

The negative electrode and the positive electrode manufactured asdescribed above were stacked with the separator formed of apolypropylene film and wound in the longitudinal direction, and theprotective tape was bonded to the outermost portion thereof, therebymanufacturing a battery element.

Finally, the manufactured battery element was inserted into a sheathmember formed of an aluminum laminate film with an aluminum foilinterposed between polyolefin, the outer edges other than one edge werethermally fused to form a bag shape, and the battery element was housedin the sheath member. Then, the electrolyte was injected into the sheathmember through the non-fused portion and then the non-fused portion ofthe sheath member was sealed air-tightly. A solution obtained by solving1.0 mol/dm³ of LiPF₆ of the lithium salt in a solvent in whichfluoroethylene carbonate (FEC) and diethyl carbonate (DEC) are mixed atthe mass ratio of 3:7 was used as the electrolyte solution. In this way,a flat type secondary battery of a 500 mAh class was manufactured.

Example B

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=95:5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

Then, as the result of surface analysis of an element distribution ofthe manufactured positive-electrode active material with the SEM-EDS, itwas seen that the element ratio of cerium Ce is 31 at. %, the ceriumelement is detected at the higher mol ratio relative to the feed amount,and the cerium Ce element exists much in the surface of thepositive-electrode active material.

95.975 wt % of the active material, 3 wt % of polyvinylidene fluoride asthe binder, 1 wt % of Ketjen black as the conductive material, and 0.025wt % of H₃PO₃ were kneaded in N-methyl pyrrolidone as the dispersionmedium to form a positive-electrode mixture slurry. The slurry wasapplied to a positive-electrode current collector formed of aluminumwith a thickness of 30 μm, dried, and compacted by the use of the rollpress machine to form a positive-electrode mixture layer, therebymanufacturing the positive electrode.

A battery was manufactured in the same way as Example A, except that thepositive electrode is used.

Example 1

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=98.5:1.5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

Then, as the result of surface analysis of an element distribution ofthe manufactured positive-electrode active material with the SEM-EDS, itwas seen that the element ratio of cerium Ce is 9 at. %, the cerium Ceelement is detected at the higher mol ratio relative to the feed amount,and the cerium Ce element exists much in the surface of thepositive-electrode active material, that is, the concentration of CeriumCe is higher in the surface than in the inside.

Then, 96 wt % of the manufactured positive-electrode active material, 3wt % of polyvinylidene fluoride as the first polymer of the binder, and1 wt % of Ketjen black as the conductive material were kneaded inN-methyl pyrrolidone as the dispersion medium to form apositive-electrode mixture slurry. The slurry was applied to apositive-electrode current collector formed of aluminum with a thicknessof 30 μm, dried, and compacted by the use of the roll press machine toform a positive-electrode mixture layer, thereby manufacturing thepositive electrode.

A negative electrode was manufactured as follows using silicon as thenegative-electrode active material and using a deposition method. Metalsilicon ground into a chip shape was pulverized up to 1 μm by the use ofa jet mill. The silicon powder was dispersed in 3 wt % of polyamide acid(polyimide precursor)/NMP solution to form a slurry shape, the slurrywas applied to an electrolyte copper foil as the negative-electrodecurrent collector, dried, and then compacted by the use of the rollpress machine. Thereafter, the resultant was subjected to heat treatmentin vacuum at 400° C. for 3 hours. As a result, the negative electrodewas manufactured. In this state, the initial composition of the slurrywas adjusted by measurement of mass so that the weight ratio of siliconand polyimide is 90:10 from the weight measurement.

The negative electrode and the positive electrode manufactured asdescribed above were stacked with the separator formed of apolypropylene film and wound in the longitudinal direction, and theprotective tape was bonded to the outermost circumferential portionthereof, thereby manufacturing a battery element.

Finally, the manufactured battery element was inserted into a sheathmember formed of an aluminum laminate film with an aluminum foilinterposed between polyolefin, the outer edges other than one edge werethermally fused to form a bag shape, and the battery element was housedin the sheath member. Then, the electrolyte was injected into the sheathmember through the non-fused portion and then the non-fused portion ofthe sheath member was sealed air-tightly. A solution obtained by solving1.0 mol/dm³ of LiPF₆ of the lithium salt in a solvent in whichfluoroethylene carbonate (FEC) and diethyl carbonate (DEC) are mixed atthe mass ratio of 3:7 was used as the electrolyte solution. In this way,a flat type secondary battery of a 500 mAh class was manufactured.

Example 2

A battery was manufactured by synthesizing the positive-electrode activematerial in the same way as Example 1, except that predetermined amountsof ultrafine powders of LiCoO₂ and CeO₂ were weighed so as to have a molratio of Co:Ce=97:3 and processed by the powder complicating machineNobilta NOB made by Hosokawa Micron Corporation.

Example 3

A battery was manufactured by synthesizing the positive-electrode activematerial in the same way as Example 1, except that predetermined amountsof ultrafine powders of LiCoO₂ and CeO₂ were weighed so as to have a molratio of Co:Ce=95:5 and processed by the powder complicating machineNobilta NOB made by Hosokawa Micron Corporation.

Example 4

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=98.5:1.5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

Then, as the result of surface analysis of an element distribution ofthe manufactured positive-electrode active material with the SEM-EDS, itwas seen that the element ratio of cerium Ce is 9.7 at. %, the cerium Ceelement is detected at the higher mol ratio relative to the feed amount,and the cerium Ce element exists much in the surface of thepositive-electrode active material.

95.95 wt % of the active material, 3 wt % of polyvinylidene fluoride asthe binder, 1 wt % of Ketjen black as the conductive material, and 0.05wt % of H₃PO₃ were kneaded in N-methyl pyrrolidone as the dispersionmedium to form a positive-electrode mixture slurry. The slurry wasapplied to a positive-electrode current collector formed of aluminumwith a thickness of 30 μm, dried, and compacted by the use of the rollpress machine to form a positive-electrode mixture layer, therebymanufacturing the positive electrode.

Then, the positive electrode was analyzed by the use of the TOF-SIMS.The peaks of fragments based on positive secondary ions of Li₄PO₄,Li₂CoPO₄, Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and negative secondaryions of PO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, and CoP₂O₆H wereobserved. This result indicates that compounds such as Li₃PO₄ andLiCoPO₄ exist in the particle surface of the positive-electrode activematerial.

The positive electrode was analyzed by the use of the TOF-SIMS asfollows. The positive electrode was embedded in resin, the section wasprocessed by the use of an argon ion milling machine, and then theTOF-SIMS analysis was performed thereon. TOF-SIMSV made by ION-TOF wasused for the TOF-SIMS analysis under the measurement conditions ofprimary ions of 197 Au⁺, ion-gun accelerating voltage of 25 keV,unbunching, radiated ion current of 0.5 pA (measured with a pulse beam),pulse frequency of 50 kHz, mass range of 1 to 200 amu, scanning range of25×25 μtm, and spatial resolution of 0.2 μm. It could be seen from theanalysis result that phosphorous compounds such as PO₃ exist to coverLiCoO₂ particles. Then, a battery was manufactured similarly to Example1, except that the positive electrode manufactured as described above isused.

Example 5

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=98.5:1.5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

95.975 wt % of the active material, 3 wt % of polyvinylidene fluoride asthe binder, 1 wt % of Ketjen black as the conductive material, and 0.025wt % of H₃PO₃ were kneaded in N-methyl pyrrolidone as the dispersionmedium to form a positive-electrode mixture slurry. The slurry wasapplied to a positive-electrode current collector formed of aluminumwith a thickness of 30 μm, dried, and compacted by the use of the rollpress machine to form a positive-electrode mixture layer, therebymanufacturing the positive electrode.

A battery was manufactured similarly to Example 1, except that themanufactured positive electrode is used.

Example 6

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=98.5:1.5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

95.99 wt % of the active material, 3 wt % of polyvinylidene fluoride asthe binder, 1 wt % of Ketjen black as the conductive material, and 0.01wt % of H₃PO₃ were kneaded in N-methyl pyrrolidone as the dispersionmedium to form a positive-electrode mixture slurry. The slurry wasapplied to a positive-electrode current collector formed of aluminumwith a thickness of 30 μm, dried, and compacted by the use of the rollpress machine to form a positive-electrode mixture layer, therebymanufacturing the positive electrode.

A negative electrode was manufactured as follows using silicon as thenegative-electrode active material and using a deposition method. Apartially-oxidized amorphous silicon layer was formed with a thicknessof 4 μm on an electrolyte copper foil of which the surface wasroughened, using the same metal silicon (purity of 99%) as used inExample 1 and using an electron beam deposition method, whileintroducing oxygen gas diluted with argon into a chamber.

A battery was manufactured similarly to Example 1, except that themanufactured positive electrode and negative electrode are used.

Example 7

A positive-electrode active material was manufactured as follows. First,predetermined amounts of ultrafine powders of LiCoO₂ and CeO₂ wereweighed so as to have a mol ratio of Co:Ce=98.5:1.5, placed in a powdercomplicating machine Nobilta NOB made by Hosokawa Micron Corporation,and then processed with the power of 4 kW for 15 minutes. A desiredpositive-electrode active material was obtained by heating the processedpowders in an air stream at 950° C. for 5 hours and then filtering thepowders with a 75 μm or less sieve.

95.99 wt % of the active material, 3 wt % of polyvinylidene fluoride asthe binder, 1 wt % of Ketjen black as the conductive material, and 0.01wt % of sulfobenzoic acid were kneaded in N-methyl pyrrolidone as thedispersion medium to form a positive-electrode mixture slurry. Theslurry was applied to a positive-electrode current collector formed ofaluminum with a thickness of 30 μm, dried, and compacted by the use ofthe roll press machine to form a positive-electrode mixture layer,thereby manufacturing the positive electrode.

A battery was manufactured similarly to Example 5, except that themanufactured positive electrode is used.

Example 8

A positive electrode was manufactured as follows. 95.9 wt % of LiCoO₂with a mean particle size of 12 μm and a specific surface area of 0.20m²/g, 3 wt % of polyvinylidene fluoride as the first polymer of thebinder, 0.05 wt % of polyvinyl pyrrolidone as the second polymer, 1 wt %of Ketjen black as the conductive material, and 0.05 wt % of methanesulfonic acid were kneaded in N-methyl pyrrolidone as the dispersionmedium to form a positive-electrode mixture slurry. The slurry wasapplied to a positive-electrode current collector formed of aluminumwith a thickness of 30 μm, dried, and compacted by the use of the rollpress machine to form a positive-electrode mixture layer, therebymanufacturing the positive electrode.

As the result of analysis using the TOF-SIMS, the same peaks ofsecondary ions fragments as Example 4 were observed.

Then, a secondary battery was manufactured similarly to Example 7,except that the positive electrode manufactured as described above isused.

Example 9

A positive electrode was manufactured as follows. 95.9 wt % of LiCoO₂with a mean particle size of 12 μm and a specific surface area of 0.20m²/g, 3 wt % of polyvinylidene fluoride as the first polymer of thebinder, 0.05 wt % of polyvinyl pyrrolidone as the second polymer, 1 wt %of Ketjen black as the conductive material, and 0.05 wt % ofsulfobenzoic anhydride were kneaded in N-methyl pyrrolidone as thedispersion medium to form a positive-electrode mixture slurry. Theslurry was applied to a positive-electrode current collector formed ofaluminum with a thickness of 30 μm, dried, and compacted by the use ofthe roll press machine to form a positive-electrode mixture layer,thereby manufacturing the positive electrode.

As the result of analysis using the TOF-SIMS, the same peaks ofsecondary ions fragments as Example 4 were observed.

Then, a secondary battery was manufactured similarly to Example 7,except that the positive electrode manufactured as described above isused.

Comparative Example A

A positive electrode was manufactured as follows. 96 wt % of LiCoO₂ witha mean particle size of 12 μm and a specific surface area of 0.20 m²/g,3 wt % of polyvinylidene fluoride as the binder, and 1 wt % of Ketjenblack as the conductive material were kneaded in N-methyl pyrrolidone asthe dispersion medium to form a positive-electrode mixture slurry. Theslurry was applied to a positive-electrode current collector formed ofaluminum with a thickness of 30 μm, dried, and compacted by the use ofthe roll press machine to form a positive-electrode mixture layer,thereby manufacturing the positive electrode.

Then, the positive electrode was analyzed using the TOF-SIMS. As aresult, the peaks of fragments based on positive secondary ions ofLi₄PO₄, Li₂CoPO₄, Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and negativesecondary ions of PO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, andCoP₂O₆H were not observed. Similarly to Example 4, as the result ofanalysis using the TOF-SIMS after embedding the positive electrode inresin and processing the section with the argon ion milling machine, itcould be seen that phosphorous compounds do not exist in the sectionalsurface. Then, a secondary battery was manufactured similarly to ExampleA, except that the positive electrode manufactured as described above isused.

Comparative Example B

A positive electrode was manufactured as follows. 95.8 wt % of LiCoO₂with a mean particle size of 12 μm and a specific surface area of 0.20m²/g, 3 wt % of polyvinylidene fluoride as the binder, 1 wt % of Ketjenblack as the conductive material, and 0.05 wt % of H₃PO₃ were kneaded inN-methyl pyrrolidone as the dispersion medium to form apositive-electrode mixture slurry. The slurry was applied to apositive-electrode current collector formed of aluminum with a thicknessof electrode 30 μm, dried, and compacted by the use of the roll pressmachine to form a positive-electrode mixture layer, therebymanufacturing the positive electrode.

Then, a secondary battery was manufactured similarly to Example A,except that the positive electrode manufactured as described above isused.

Comparative Example 1

A positive electrode was manufactured as follows. 96 wt % of LiCoO₂ witha mean particle size of 12 μm and a specific surface area of 0.20 m²/g,3 wt % of polyvinylidene fluoride as the binder, and 1 wt % of Ketjenblack as the conductive material were kneaded in N-methyl pyrrolidone asthe dispersion medium to form a positive-electrode mixture slurry. Theslurry was applied to a positive-electrode current collector formed ofaluminum with a thickness of 30 μm, dried, and compacted by the use ofthe roll press machine to form a positive-electrode mixture layer,thereby manufacturing the positive electrode.

Then, the positive electrode was analyzed using the TOF-SIMS. As aresult, the peaks of fragments based on positive secondary ions ofLi₄PO₄, Li₂CoPO₄, Li₂CoPH₂O₄, Li₃CoPO₄, and Li₃CoPO₄H and negativesecondary ions of PO₂, LiP₂O₄, Co₂PO₄, CoP₂O₅, CoP₂O₅H, CoP₂O₆, andCoP₂O₆H were not observed. Similarly to Example 4, as the result ofanalysis using the TOF-SIMS after embedding the positive electrode inresin and processing the section with the argon ion milling machine, itcould be seen that phosphorous compounds do not exist in the sectionalsurface. Then, a secondary battery was manufactured similarly to Example1, except that the positive electrode manufactured as described above isused.

Comparative Example 2

A positive electrode was manufactured as follows. 95.8 wt % of LiCoO₂with a mean particle size of 12 μm and a specific surface area of 0.20m²/g, 3 wt % of polyvinylidene fluoride as the binder, 1 wt % of Ketjenblack as the conductive material, and 0.2 wt % of H₃PO₃ were kneaded inN-methyl pyrrolidone as the dispersion medium to form apositive-electrode mixture slurry. The slurry was applied to apositive-electrode current collector formed of aluminum with a thicknessof 30 μm, dried, and compacted by the use of the roll press machine toform a positive-electrode mixture layer, thereby manufacturing thepositive electrode.

Then, a secondary battery was manufactured similarly to Example 1,except that the positive electrode manufactured as described above isused.

Comparative Example 3

A positive electrode was manufactured as follows. 95.8 wt % of LiCoO₂with a mean particle size of 12 μm and a specific surface area of 0.20m²/g, 3 wt % of polyvinylidene fluoride as the binder, 1 wt % of Ketjenblack as the conductive material, and 0.05 wt % of H₃PO₃ were kneaded inN-methyl pyrrolidone as the dispersion medium to form apositive-electrode mixture slurry. The slurry was applied to apositive-electrode current collector formed of aluminum with a thicknessof 30 μm, dried, and compacted by the use of the roll press machine toform a positive-electrode mixture layer, thereby manufacturing thepositive electrode.

Then, a secondary battery was manufactured similarly to Example 1,except that the positive electrode manufactured as described above isused. Estimation of High-Temperature Storage Characteristic

The secondary battery manufactured as described above was charged withconstant current of 0.2 C until the battery voltage reaches 4.2 V andthen charged with constant voltage of 4.2 V until the current valuereaches 0.01 C. Then, constant current discharge was made with aconstant current density of 0.2 C until the battery voltage reaches 2.5V. The secondary battery charged up to 4.2 V after the first chargingand discharging was stored in a constant-temperature bath of 85° C. for12 hours. Thereafter, the increase in thickness was calculated from thepack thicknesses measured before the storage and after the storage, andthe increase in thickness was used as a criterion of the amount ofgenerated gas due to the positive electrode.

The variation rates in thickness of the examples and the comparativeexamples were calculated by the use of the following expression usingthe calculated increase in thickness. The results are shown in Table 1.In the variation rates in thickness, the variation rate in packthickness of Comparative Example 1 was assumed as being 100%.

Variation rate in thickness of aluminum laminate pack (%)=[(increase inpack thickness after storage at 85° C. for 12 hours in respectiveExamples (mm))/(increase in pack thickness after storage at 85° C. for12 hours in Comparative Example A (mm))]×100

Comparison of Nature of Positive-Electrode Mixture Slurry

Between the examples and the comparative examples, the additionalamounts of dispersion medium (N-methyl pyrrolidone) required forcompletion of the optimal-viscosity slurry at the time of manufacturingthe positive-electrode mixture slurry were compared using the valuescalculated by the use of the following expression.

Ratio of dispersion medium (%)=[(amount of N-methyl pyrrolidonenecessary for manufacturing mixture slurry in respective Examples(g))/(amount of N-methyl pyrrolidone necessary for Comparative Example 1(g))]×100

The peeling resistance between the positive-electrode mixture layer andthe current collector was measured and compared. The coated and pressedelectrode was cut with a width of 25 mm, a double-sided tape wasattached thereto, and the foil was peeled, whereby the peelingresistance was measured. The peeling resistance was compared using thevalues calculated by the following expression.

Ratio of peeling resistance (%)=[(peeling resistance of positiveelectrode manufactured in respective Examples (N/mm))/(peelingresistance of positive electrode manufactured in Comparative Example A(N/mm))]×100

TABLE 1 Negative Variation Addition to Ratio of electrode rate inPositive-electrode active mixture Mol ratio of dispersion Ratio ofpeeling manufacturing thickness of material slurry P, S/Co (at. %)medium (%) resistance (%) method battery (%) Example A LiCo: 0.95, Ce:0.0502 — — 99 100 Graphite 85 Example B LiCo: 0.95, Ce: 0.0502 H₃PO₃0.03 106 98 Graphite 69 Example 1 LiCo: 0.985, Ce: 0.01502 — — 99 100Application 85 Example 2 LiCo: 0.97, Ce: 0.0302 — — 101 99 Application74 Example 3 LiCo: 0.95, Ce: 0.0502 — — 102 100 Application 61 Example 4LiCo: 0.95, Ce: 0.0502 H₃PO₃ 0.06 118 95 Application 20 Example 5 LiCo:0.95, Ce: 0.0502 H₃PO₃ 0.03 106 98 Application 40 Example 6 LiCo: 0.95,Ce: 0.0502 H₃PO₃ 0.01 102 100 deposition 52 Example 7 LiCo: 0.95, Ce:0.0502 sulfo-benzoic 0.02 105 106 deposition 65 anhydride ComparativeExample A LiCoO₂ No addition — 100 100 Graphite 100 Comparative ExampleB LiCoO₂ No addition 0.06 117 82 Graphite 66 Comparative Example 1LiCoO₂ No addition — 100 100 Application 100 Comparative Example 2LiCoO₂ H₃PO₃ 0.3  125 15 Application 25 Comparative Example 3 LiCoO₂H₃PO₃ 0.06 118 81 Application 49

It can be seen from Table 1 that it is possible to suppress the increasein thickness of the laminate pack due to the high-temperature storage inExamples 1 to 7 in comparison with Comparative Example 1. It can be seenthat the increase in thickness can be suppressed in a sample in whichcerium Ce is attached to the surface of the active material, but thegeneration of gas due to the reaction of the charged positive electrodeand the electrolyte solution can be further suppressed in the battery inwhich the compounds containing phosphorous P or sulfur S are added tothe positive-electrode slurry.

However, by adding only the compounds containing phosphorous P or sulfurS to the positive electrode as described in Comparative Examples 2 and3, a large amount of dispersion medium is required for manufacturing thepositive-electrode mixture slurry and the peeling resistance afterpressing is markedly deteriorated. On the contrary, in Examples 4 to 9,the active material having been subjected to the surface treatment withcerium is used. Accordingly, even when the addition of the compoundscontaining phosphorous P or sulfur S is suppressed to the minimum, it ispossible to obtain the satisfactory high-temperature storagecharacteristic. As a result, it is possible to reduce the amount ofdispersion medium and to improve the peeling resistance after pressing.

Accordingly, according to the embodiment, it is possible to provide abattery employing a high-capacity negative electrode, with ahigh-temperature storage characteristic and excellent in productivityand durability of the positive electrode.

Although the present application has been described in detail withreference to the embodiments and the examples, the present applicationis not limited to the embodiments and the examples, but may be modifiedin a variety of suitable ways.

For example, the numerical values described in the embodiments and theexamples are only examples and numerical values different therefrom maybe used as needed.

Although it has been described in the embodiments and the examples thatthe present application is applied to the flat and cylindrical secondarybatteries, the present application may be similarly applied to secondarybatteries of an angular type, a button type, a thin type, a large-scaledtype, and a laminate type. The present application may be applied toprimary batteries, as well as the secondary batteries.

Although it has been described in the embodiments and the examples thatthe present application is applied to the secondary batteries employinga single body of graphite or silicon Si as the negative-electrode activematerial, the positive electrode according to the present applicationmay be similarly applied to secondary batteries employing as thenegative-electrode active material an alloy of silicon Si, a mixed bodyof silicon Si and carbon C, or a single body or a compound including anelement such as tin Sn or germanium Ge.

In the embodiments, lithium composite oxide having the layered structureexpressed by the following formula (2) or lithium composite phosphatehaving the phosphate structure expressed by the following formula (3)may be used as the lithium composite oxide in the embodiments.

Li_(p)Ni_((1−q−r))Mn_(q)M_(1r)O_((2−y))X_(z)   (2)

Here, M1 represents at least one element selected of Group 2 to Group 15other than Ni and Mn. X represents at least one element selected fromGroup 16 and Group 17 other than oxygen O. In addition, p, q, y, and zrepresent values satisfying 0≦p≦1.5, 0≦q≦1.0, 0≦r≦1.0, −0.10≦y≦0.20, and0≦z≦0.2, respectively.

Li_(a)M2_(b)PO₄   (3)

Here, M2 represents at least one element selected from Groups 2 to 15,and a and b represent values satisfying 0≦a≦2.0 and 0.5≦b≦2.0,respectively.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations, and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

It should be understood that various changes and modifications to thepresently preferred embodiments described herein will be apparent tothose skilled in the art. Such changes and modifications can be madewithout departing from the spirit and scope of the present subjectmatter and without diminishing its intended advantages. It is thereforeintended that such changes and modifications be covered by the appendedclaims.

1. A positive-electrode active material for a lithium-ion secondarybattery comprising an average composition expressed by the followingformula (1):Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1) wherein M represents atleast one element selected from the group consisting of boron B,magnesium Mg, aluminum Al, silicon Si, phosphorous P, sulfur S, titaniumTi, chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu,zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag,tungsten W, indium In, tin Sn, lead Pb, and antimony Sb, X represents ahalogen element, and x, y, z, a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1,0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0, respectively, wherein theconcentration of cerium Ce is higher in a vicinity of a surface of thepositive-electrode active material as composed to an inside thereof. 2.The positive-electrode active material according to claim 1, wherein thepositive-electrode active material is particle shaped and at least in apart of the surface has a surface layer where the concentration ofcerium Ce is higher than the inside of the particle.
 3. Thepositive-electrode active material according to claim 1, wherein thecontent of cerium Ce in the vicinity of the surface ranges from 5 at. %to 50 at. % with respect to cobalt Co.
 4. A positive electrode for alithium-ion secondary battery comprising a positive-electrode mixturelayer and a current collector, the positive-electrode mixture layerincluding a positive-electrode active material, a conductive material,and a binder, wherein the positive-electrode active material has anaverage composition expressed by the following formula (1):Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1) wherein M represents atleast one element selected from the group consisting of boron B,magnesium Mg, aluminum Al, silicon Si, phosphorous P, sulfur S, titaniumTi, chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu,zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag,tungsten W, indium In, tin Sn, lead Pb, and antimony Sb, X represents ahalogen element, and x, y, z, a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1,0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0, respectively, wherein theconcentration of cerium Ce is higher in a vicinity of a surface of thepositive-electrode active material as compared to an inside thereof, andwherein a compound including sulfur S and/or phosphorous P is includedin the positive-electrode mixture layer.
 5. The positive electrodeaccording to claim 4, wherein the content of sulfur S and/or phosphorousP in the positive-electrode mixture layer is equal to or greater than0.02 at. % and less than 0.10 at. % with respect to cobalt Co.
 6. Thepositive electrode according to claim 4, wherein the compound includingsulfur S and/or phosphorous P in the positive-electrode mixture layer ishigher in concentration in the vicinity of the surface of thepositive-electrode active material than in the inside thereof.
 7. Thepositive electrode according to claim 4, wherein the positive electrodeis used along with a negative electrode which intercalate anddeintercalate lithium ions and which includes at least one elementselected from the group consisting of silicon Si, tin Sn, and germaniumGe as a metal material that can occlude and desorb lithium ions.
 8. Amethod of manufacturing a positive electrode for a lithium-ion secondarybattery including a positive-electrode mixture layer, which includes apositive-electrode active material, a conductive material, and a binder,and a current collector, the method comprising: providing thepositive-electrode active material has an average composition expressedby the following formula (1):Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1) wherein M represents atleast one element selected from the group consisting of boron B,magnesium Mg, aluminum Al, silicon Si, phosphorous P, sulfur S, titaniumTi, chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu,zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag,tungsten W, indium In, tin Sn, lead Pb, and antimony Sb, X represents ahalogen element, and x, y, z, a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1,0.5<z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0, respectively, and wherein theconcentration of cerium Ce is higher in a vicinity of a surface of thepositive-electrode active material as compared to an inside thereof; andadding the compound containing sulfur S and/or phosphorous P to apositive-electrode mixture slurry used to form the positive-electrodemixture layer.
 9. A lithium-ion secondary battery comprising: a positiveelectrode, which includes a positive-electrode mixture layer having apositive-electrode active material, a conductive material, and a binder,and a current collector; a negative electrode; and an electrolyte,wherein the positive-electrode active material has an averagecomposition expressed by the following formula (1):Li_(x)Co_(1−y−z)M_(y)Ce_(z)O_(b−a)X_(a)   (1) wherein M represents atleast one element selected from the group consisting of boron B,magnesium Mg, aluminum Al, silicon Si, phosphorous P, sulfur S, titaniumTi, chromium Cr, manganese Mn, iron Fe, cobalt Co, nickel Ni, copper Cu,zinc Zn, gallium Ga, yttrium Y, zirconium Zr, molybdenum Mo, silver Ag,tungsten W, indium In, tin Sn, lead Pb, and antimony Sb, X represents ahalogen element, and x, y, z, a, and b satisfy 0.2<x≦1.2, 0≦y≦0.1,0.5≦z≦5.0, 1.8≦b≦2.2, and 0≦a≦1.0, respectively, wherein theconcentration of cerium Ce is higher in a vicinity of a surface of thepositive-electrode material as compared to an inside thereof, andwherein a compound including sulfur S and/or phosphorous P is includedin the positive-electrode mixture layer.
 10. The lithium-ion secondarybattery according to claim 9, wherein the electrolyte includes a cyclicester carbonate or a chained ester carbonate in which all or a part ofhydrogen is fluorinated.